The Collaborative International Dictionary of English v.0.48:

Principal \Prin"ci*pal\, a. [F., from L. principalis. See
   Prince.]
   1. Highest in rank, authority, character, importance, or
      degree; most considerable or important; chief; main; as,
      the principal officers of a Government; the principal men
      of a state; the principal productions of a country; the
      principal arguments in a case.
      [1913 Webster]

            Wisdom is the principal thing.        --Prov. iv. 7.
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   2. Of or pertaining to a prince; princely. [A Latinism]
      [Obs.] --Spenser.
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   Principal axis. See Axis of a curve, under Axis.

   Principal axes of a quadric (Geom.), three lines in which
      the principal planes of the solid intersect two and two,
      as in an ellipsoid.

   Principal challenge. (Law) See under Challenge.

   Principal plane. See Plane of projection
      (a), under Plane.

   Principal of a quadric (Geom.), three planes each of which
      is at right angles to the other two, and bisects all
      chords of the quadric perpendicular to the plane, as in an
      ellipsoid.

   Principal point (Persp.), the projection of the point of
      sight upon the plane of projection.

   Principal ray (Persp.), the line drawn through the point of
      sight perpendicular to the perspective plane.

   Principal section (Crystallog.), a plane passing through
      the optical axis of a crystal.
      [1913 Webster]